Generally, in this kind of electric discharge machining apparatus, an electrode for machining and a workpiece to be machined are arranged in machining liquid and voltage is applied therebetween them to develop electric discharge to make a hole in the workpiece to be machined by electric discharge machining. In order to make a hole of a desired shape while keeping a stable machining state, the electric discharge machining apparatus is provided with a driving unit for adjusting the position of the electrode for machining or the workpiece to be machined.
A command to the driving unit adjusts the position of the electrode for machining or the workpiece to be machined according to the deviation of an inter-electrode voltage, which expresses the relative position of the electrode for machining to the workpiece to be machined, from a target value of this inter-electrode voltage to thereby control the position so as to realize a stable machining state. However, the machining state of the workpiece to be machined by means of the electrode for machining may fluctuate irregularly. Hence, in order to keep the machining state stably, the high-speed responsivity of an XYZ driving unit that controls and drives the electrode for machining in directions of X-axis, Y-axis, and Z-axis to position the electrode for machining becomes important.
Then, in a conventional electric discharge machining apparatus provided with the XYZ driving unit, there has been already known an apparatus that is so constructed as to drive the electrode for machining in a radial direction and in a thrust direction in a non-contact manner by electric magnets in order to improve the high-speed responsivity of the XYZ driving unit to thereby achieve the high-speed responsivity in the directions of the X axis, the Y axis, and the Z axis of the electrode for machining, thereby being able to realize the enhancement of the speed of machining and the enhancement of the accuracy of machining (for example, refer to patent document 1).
Moreover, as another conventional example has been already known an electric discharge machining apparatus having a construction in which: the upper end of a pipe electrode is held by an electrode holder; the electrode holder is chucked by a chuck fixed to the tip of main shaft of a machining machine; and the pipe electrode consumed and shortened by electric discharge machining can be replaced together with the electrode holder; and the pipe electrode held by the electrode holder is made longer, thereby being able to respond to a long time machining (for example, refer to patent document 2).
Further, as an electric discharge machining apparatus according to another conventional example has been already known an apparatus having a construction in which: a small-diameter wire electrode fed out from an electrode supply reel as electrode feeding means is pressed and pinched by two rollers (feeding roller and clamping roller); and one of the rollers is rotated and driven by a servomotor to thereby feed the small-diameter wire electrode continuously (for example, refer to patent document 3).
Still further, as another electrode feeding means of this kind has been already known means having a construction in which a small-diameter wire electrode is pinched by two rollers and is fed out by the amount of consumption by rotating these rollers by a manual operation (for example, refer to patent document 4).    [Patent document 1] International Publication No. WO 02/024389 A1 (page 12 to 15, FIG. 1, FIG. 2)    [Patent document 2] JP-A-08-290332 (pages 6 and 7, FIG. 1, FIG. 2)    [Patent document 3] JP-A-20001-38532 (page 5, FIG. 1)    [Patent document 4] European Patent EP 0826455 B1 (page 2, FIG. 1, FIG. 2)
Because the conventional electric discharge machining apparatuses are constructed in the above-mentioned manner, they present the following various problems.
In the electric discharge machining apparatus disclosed in the patent document 1, high-speed responsivity in the directions of the X-axis, Y-axis, and Z-axis can be achieved. However, when electric discharge machining is performed by means of the electric discharge machining apparatus disclosed in the patent document 1 by using, for example, a tungsten small-diameter wire having a diameter of 0.2 mm or less as an electrode for machining, because the electrode of tungsten small-diameter wire is consumed by machining, in order to make a hole of a desired depth, the tungsten small-diameter wire of a length meeting the amount of estimation of consumption needs to be fed out. However, the electric discharge machining apparatus disclosed in the patent document 1 does not describe anything about the function of feeding out the above-mentioned small-diameter wire automatically. Moreover, according to the electric discharge machining apparatus disclosed in the patent document 1, the speed of machining the workpiece to be machined by the electric discharge machining can be increased but it takes time to correct the consumption of small-diameter wire electrode when the small-diameter wire electrode is used. Hence, this raises a problem that in order to shorten total machining time, an automatic supply mechanism of the small-diameter wire electrode is required.
In the electric discharge machining apparatus disclosed in the patent document 2, because a slender low-rigidity pipe electrode having a long length is used, the pipe electrode becomes a kind of spring. When an electrode replacing mechanism for replacing such a pipe electrode and its electrode holder in their entirety is applied to the electric discharge machining apparatus disclosed in the patent document 1, the high-speed-response driving ability of the pipe electrode is impaired. This raises a problem that the sufficient effect of enhancing the speed and accuracy of electric discharge machining can not be expected. Moreover, a new pipe electrode is long in length and hence is large in resistance, but because the pipe electrode is consumed and shortened by the electric discharge machining, the pipe electrode is gradually decreased also in resistance. Hence, even though the electric conditions at the time of electric discharge machining are set at the same settings, a machining current is gradually increased during the electric discharge machining. That is, in the case of making a hole by means of an electric discharge machining apparatus of an electrode replacement type, the diameter of a hole made by the electric discharge machining is small in a state where the pipe electrode is new, but the diameter of the hole is increased as the resistance of the pipe electrode is decreased by consumption at the time of the electric discharge machining. Hence, there is presented a problem that the quality of machining cannot be kept uniformly.
In the electric discharge machining apparatus disclosed in the patent document 3, the rollers pressing and pinching the small-diameter wire electrode are rotated and driven by the servomotor and hence can supply the small-diameter wire electrode automatically and continuously. However, there is a possibility that the small-diameter wire electrode will be deformed, for example, in an ellipsoidal shape by a force that the rollers applies to the small-diameter wire electrode. Moreover, in an apparatus structure like this, the small-diameter wire electrode cannot be rotated during machining and hence the cross-sectional shape of small-diameter electrode deformed as described above becomes the shape of a hole to be machined. Hence, there is presented a problem that it is impossible to make a hole of high circularity.
In the electric discharge machining apparatus disclosed in the patent document 4, the small-diameter wire electrode is fed out by a manual operation and the operation of feeding the small-diameter wire electrode automatically is never described. For this reason, an operator needs to always control a machining state and needs to manually feed out the small-diameter wire electrode by the amount of consumption, which presents a problem that an electric discharge machining apparatus cannot be fully automated. Moreover, because the small-diameter wire electrode applied to this electric discharge machining apparatus has a diameter of 0.1 mm, there is presented a problem that the electric discharge machining of making a fine hole by using a wire having a small diameter not larger than the diameter cannot be continuously performed.
This invention has been made to solve the above-mentioned problems. The object of this invention is to provide a high-reliability electric discharge machining apparatus that: can feed out a small-diameter wire electrode automatically in a short time without deforming the small-diameter wire electrode in electric discharge machining using a hard-to-handle conducting small-diameter wire as an electrode; can prevent the buckling and vibration of a small-diameter wire electrode to be fed out and can perform the electric discharge machining of making a hole of high circularity automatically, continuously, and efficiently; can shorten the time required to replace the small-diameter wire electrode because of consumption to a large extent; and hence can shorten the time required to prepare machining at the time of making a hole by the electric discharge machining and can produce improvements in machining speed and in machining accuracy.